Manufacturers of vehicles that employ internal combustion engines, more particularly diesel engines, are under increased pressure to comply with current and future emission standards for the release of nitrogen oxides (NOX), particularly nitrogen monoxide (NO), as well as unburned and partially oxidized hydrocarbons (HC), carbon monoxide (CO), particulate matter (PM), and other pollutants, such as hydrogen sulfide (H2S) and ammonia (NH3). In order to reduce the pollutant emissions of a diesel engine, the latter are typically operated with an exhaust gas after-treatment system through which the exhaust gas from the diesel engine flows.
Exhaust gas after-treatment systems typically include one or more after-treatment devices, such as oxidation catalysts, NOX abatement devices, particulate filters and sulfur traps. These after-treatment devices generally require certain conditions to exist in the engine exhaust gas in order to perform optimally. More specifically, NOX abatement devices and oxidation catalysts, for example, have a relatively narrow temperature window within which the devices are activated, regenerated or operate with high conversion efficiency. Periodically, after-treatment devices require heating beyond that provided by the exhaust gas to achieve the desired operating temperature.
Another condition that is typically required in an exhaust gas after-treatment system is the presence of sufficient oxygen to facilitate oxidation of pollutant emissions. Additionally, diesel particulate filters periodically require a relatively high concentration of oxygen in the exhaust gas to facilitate regeneration of the particulate filter.
Prior to the present invention, the required exhaust gas conditions could not always be achieved during normal operation of the engine. More particularly, the exhaust gas temperature could only be influenced to a certain degree by the combustion process without the use of a source of supplemental heat, such as an electric heater in the exhaust-gas stream. Also, during certain engine operating conditions, it was difficult to generate the needed high levels of oxygen in the exhaust gas required by the oxidation catalysts and particulate filters.